1. Field of the Invention
This invention relates in general to semiconductor device processing, and more particularly to an effective method for optimizing the process used to outgas a layer of spin-on-glass.
2. Description of the Prior Art
Spin-on-glasses (SOGs) are commonly used in the manufacture of integrated circuits in order to achieve planarization of the surface. As the integrated circuit gets built up, layer by layer, its surface becomes increasingly uneven. This lack of planarity reduces the degree of precision with which each succeeding pattern (into which the most recently deposited layer will be shaped) can be aligned with respect to the pattern that preceded it.
SOGs are useful materials for achieving planarization since they may be applied to a surface in liquid form (i.e. uncured) and then converted to solid form, in situ, through suitable heat treatment (i.e. cured). While in liquid form, the SOG flows into all the depressions in the surface to which it was applied, presenting a planar surface of its own once flow has ceased. The solid material to which it is converted after heat treatment is essentially silicon dioxide which is compatible with the other materials that form the integrated circuit well as being compatible with any subsequent processing steps yet be performed on the integrated circuit.
Once creation of the SOG layer has been completed, manufacture of the integrated circuit continues with the application of additional layers of material. It was observed as part of manufacturing experience that when part of a previously deposited SOG layer was exposed as a result of, for example, etching away adjacent layers that had previously covered the SOG layer, there was a tendency for such exposed SOG surfaces to undergo ougassing under conditions of moderate heating.
For example, if via holes, etched between two levels in the integrated circuit, passed through a layer of SOG, the SOG surfaces exposed inside the via holes could sometimes emit previously trapped moisture while a layer of metal was being deposited onto the inside of the via holes. This emitted moisture reacted with the metal as it was being deposited, resulting in high contact resistance between the freshly deposited metal layer and the surface of a metal layer at the bottom of the via hole.
It has been found that an effective solution to this problem is to briefly heat the integrated circuit in vacuum prior to the deposition of the metallic layer so as to clear the exposed SOG surfaces of moisture. As in all integrated circuit manufacturing procedures, the lower the temperature and the shorter the time for which this degassing procedure could be performed, the better. The problem that had to be solved, therefore, was how to optimize time and temperature for SOG outgassing.
Prior to the development of the present invention, the optimum time and temperature for SOG outgassing were determined by performing a series of experiments and then using the results to set fixed values for these two quantities. From then on, the same time and temperature were used each time SOG degassing was needed. This approach suffers from several disadvantages. First, the amount of outgassing that will occur varies from one sample of SOG to the next. This could be because of variations in the thickness of the SOG or in the moisture retention properties of a given SOG sample. Second, the amount of outgassing associated with even the same sample and thickness of SOG will depend on the prior treatment received by the SOG, factors such as type of etchant used to expose the SOG, duration of etching, time that the exposed SOG was open to the atmosphere, etc., all playing a role.
Accordingly, a method was needed for quantitatively monitoring the outgassing of the SOG while it occurred and then terminating the heat treatment as soon as outgassing had ceased. A method for detecting the presence of water in semiconductor devices has been described by Andrews, Lifshitz, and Smolinsky in U.S. Pat. No. 4,938,847 (July 1990). This method comprises applying high voltage across the region that is to be monitored and then relating the resulting leakage current to the concentration of trapped water molecules. This method is not suitable for dealing with the SOG outgassing problem since the geometries involved (thin layers deep inside via holes) preclude the use of voltage probes. Also, the method measures moisture that has been left behind rather than what was emitted. The purpose of the outgassing heat treatment is to ensure that that no moisture will be present during metal film deposition, not necessarily to drive out all moisture that may be present in the SOG.
It was therefore an object of the present invention to be able to determine precisely when outgassing had ceased and to then use this information to limit the duration of the outgassing process to the minimum that was needed. This has been achieved through use of a residual gas analyzer attached to a control unit, as will be described below.